1. Field of the Invention
The invention relates to micromachined thermal flowmeters for measuring the flow rate of a flowing fluid, e.g., a liquid or a gas.
2. Description of the Prior Art
In general, a micromachined thermal flow meter is operated based on the principle of a well-known, hot-wire anemometer and fabricate by using modern silicon integrated circuit (IC) technology. Such a flowmeter offers many advantages including small size, low input power, high sensitivity, fast response, ability for integration, and easiness for batch production. The flowmeters have found an ever-increasing variety of applications such as, for instance, process control in the chemical or semiconductor industries, air conditioning and building control, combustion control in engines and furnaces, and medical measurements.
Over the last ten years several types of the micromachined thermal flowmeters have been developed.
In the first type, a thermopile gas flowmeter uses a thin single crystal membrane structure micromachined in a silicon substrate for providing high thermal isolation, as shown FIG. 1. A heating resistor (104) is disposed in the central region of the membrane (102). A thermopile consists of 20 aluminum/polysilicon thermocouples (105) placed on the membrane (102). The "hot" contacts are positioned near the heating resistor (104) at the tip of the membrane (102), the "cold" contacts are located on the bulk silicon (101). The flowmeter also comprises a passivation layer (103) and a metallization pattern including bonding pads (106).
This type of flowmeters suffers the following problems:
(1) The thin membrane of the flowmeter is easy to damage under the conditions of higher air flow loading and bombardment of particulate matter. PA0 (2) The fluid flow to be measured is easy to be disturbed by the opening on the surface of the membrane adapted to allow the fluid passing over. PA0 (3) The flowmeter can not be used for liquid because the liquid filled in the opening would reduce the thermal resistance between the cantilever beam and the bulk silicon. PA0 (4) The flowmeter can not be used in corrosion environment, because the back side of the thin membrane has no protecting layer thereon.
In the second type, a flowmeter has an air flow opening micromachined in a silicon substrate (201) by anisotropic chemical etching, and bridged by two beams (202), as shown in FIGS. 2A, 2B. Each bean has a nickel film resistor (204) along its length, electrically isolated from the underlying silicon by a SiO.sub.2 layer (203) and passivated by a Si.sub.3 N.sub.4 layer (205), but thermally closely coupled to it. Aluminum leads make contact with these resistors and connect to four bonding pads (206) on one edge of the chip. One beam (202) is heated via its resistor by means of a control circuit. The other beam (202) is unheated and serves as an ambient-temperature reference for temperature compensation. this type of flowmeters, the above mentioned problems (2), (3) and (4) remain to be solved. In addition, large cross-section area of the beams degrades the performance of the flowmeters such as sensitivity and response time.
In third type, a flowmeter, as shown in FIG. 3, is made of a silicon substrate (301) having a central circular region and an outre annular region on one side and a cap (306) having two cavities (307) and (308). A heating element (303) is disposed in the central circular region and at least one thermometer component (304) is disposed in the outer annular region of the silicon substrate (301). The two regions are insulated from each other by a ring region of oxidized porous silicon (305). The flowmeter is adapted to receive the flow of fluid over side of the substrate which has a micromachined cavity (302).
This type of flowmeters also has several problems.
Firstly, the manufacturing process of the flowmeters involves two substrate-processing and then bonding the two substrates together with specific alignment and bonding tools. This complicated process increases cost greatly.
Secondly, the oxidized porous silicon has thermal expansion characteristics that are different from the silicon. Due to the thermal stress, the devices disposed near the oxidized porous silicon are easy to degrade if the change in the operation temperature is too large.
Thirdly, the recesses of the cap substrate prevent the device substrate from thinning out to a small thickness. If the thickness of the device substrate is less than the depth of oxidized porous silicon region, the lateral thermal isolation between the central region and the outer annular region can not be realized very well.
Fourthly, since the cap substrate covers the front surface of the device substrate, it is difficult to adapt an electrical connection to the external circuit.